Re: [PATCH 2/2] Btrfs: improve fsync by filtering extents that we want

[Josef Bacik <jbacik@fusionio.com>]

On Mon, Aug 27, 2012 at 10:52:20AM -0600, Liu Bo wrote:
> This is based on Josef's "Btrfs: turbo charge fsync".
> 
> The above Josef's patch performs very good in random sync write test,
> because we won't have too much extents to merge.
> 
> However, it does not performs good on the test:
> dd if=/dev/zero of=foobar bs=4k count=12500 oflag=sync
> 
> The reason is when we do sequencial sync write, we need to merge the
> current extent just with the previous one, so that we can get accumulated
> extents to log:
> 
> A(4k) --> AA(8k) --> AAA(12k) --> AAAA(16k) ...
> 
> So we'll have to flush more and more checksum into log tree, which is the
> bottleneck according to my tests.
> 
> But we can avoid this by telling fsync the real extents that are needed
> to be logged.
> 
> With this, I did the above dd sync write test (size=50m),
> 
>          w/o (orig)   w/ (josef's)   w/ (this)
> SATA      104KB/s       109KB/s       121KB/s
> ramdisk   1.5MB/s       1.5MB/s       10.7MB/s (613%)
> 
> Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> ---
>  fs/btrfs/extent_map.c |   20 ++++++++++++++++++++
>  fs/btrfs/extent_map.h |    2 ++
>  fs/btrfs/inode.c      |    1 +
>  fs/btrfs/tree-log.c   |    6 +++---
>  4 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
> index 1fe82cf..ac606f0 100644
> --- a/fs/btrfs/extent_map.c
> +++ b/fs/btrfs/extent_map.c
> @@ -203,6 +203,8 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
>  			em->block_start = merge->block_start;
>  			merge->in_tree = 0;
>  			if (merge->generation > em->generation) {
> +				em->mod_start = em->start;
> +				em->mod_len = em->len;

Shouldn't this be

em->mod_start = merge->start;
em->mod_len += merge_len;

>  				em->generation = merge->generation;
>  				list_move(&em->list, &tree->modified_extents);
>  			}
> @@ -222,6 +224,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
>  		rb_erase(&merge->rb_node, &tree->map);
>  		merge->in_tree = 0;
>  		if (merge->generation > em->generation) {
> +			em->mod_len = em->len;

And this should be em->mod_len += em->len?

Thanks,

Josef